Depending on the context, you could consider hustler. This is the exact and precise word to use if someone is playing pool for money: they pretend to barely know the game, even lose a few small bets to you, then make a big bet and win dramatically. However, the word hustler has some other meanings, so if you try to apply to some context other than playing pool for money, it might be misinterpreted.

If the person’s reason for hiding their wisdom and experience is not to con or hustle you, then there is probably not a good single word for it. False, artificial, feigned or fake1 innocence, inexperience, or naïveté are probably the phrases you could use.

A disingenue, particularly if you are intending to use this to describe a girl. Disingenuous is close, but not quite satisfactory. It does not denote a discrimination of its object, which would be a girl who attempts to appear innocent. This is the opposite of an ingenue, defined as an "innocent, unworldly girl".

Another pop culture reference (with a connotation for more flamboyance or outlandishness) would be Zaphod Beeblebrox. Quoting the source:

“One of the major difficulties Trillian experienced in her relationship with Zaphood was learning to distinguish between him pretending to be stupid just to get people off their guard, pretending to be stupid because he couldn't be bothered to think and wanted someone else to do it for him, pretending to be outrageously stupid to hide the fact that he actually didn’t understand what was going on, and really being genuinely stupid. He was renowned for being amazingly clever and quite clearly was so—but not all the time, which obviously worried him, hence, the act. He preferred people to be puzzled rather than contemptuous.”

He was briefly the President of the Galaxy (a role that involves no power whatsoever, and merely requires the incumbent to attract attention so no one wonders who's really in charge, a role for which Zaphod was perfectly suited).

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